1. Field of the Invention
The present invention relates to a light-emitting element including, as an emission center, an organometallic complex capable of converting a triplet excited state into light emission. The present invention also relates to the organometallic complex. In addition, the present invention relates to a light-emitting device, an electronic device, and a lighting device each using the light-emitting element.
2. Description of the Related Art
In recent years, a light-emitting element using a light-emitting organic compound or an inorganic compound as a light-emitting material has been actively developed. In particular, a light-emitting element called an EL (electroluminescence) element has attracted attention as a next-generation flat panel display element because it has a simple structure in which a light-emitting layer containing a light-emitting material is provided between electrodes, and characteristics such as feasibility of being thinner and more lightweight and responsive to input signals and capability of driving with direct current at a low voltage. In addition, a display using such a light-emitting element has a feature that it is excellent in contrast and image quality, and has a wide viewing angle. Further, since such a light-emitting element is a plane light source, the light-emitting element is considered applicable to a light source such as a backlight of a liquid crystal display and lighting.
In the case where the light-emitting substance is an organic compound having a light-emitting property, the emission mechanism of the light-emitting element is a carrier-injection type. That is, by applying a voltage with a light-emitting layer interposed between electrodes, electrons and holes injected from electrodes recombine to make the light-emitting substance excited, and light is emitted when the excited state returns to a ground state. There are two types of the excited states which are possible: a singlet excited state (S*) and a triplet excited state (T*). In addition, the statistical generation ratio thereof in a light-emitting element is considered to be an S*-to-T* ratio of 1:3.
In general, the ground state of a light-emitting organic compound is a singlet state. Light emission from a singlet excited state (S*) is referred to as fluorescence where electron transition occurs between the same multiplicities. In contrast, light emission from a triplet excited state (T*) is referred to as phosphorescence where electron transition occurs between different multiplicities. Here, in a compound emitting fluorescence (hereinafter referred to as fluorescent compound), in general, phosphorescence is not observed at room temperature, and only fluorescence is observed. Accordingly, the internal quantum efficiency (the ratio of generated photons to injected carriers) in a light-emitting element using a fluorescent compound is assumed to have a theoretical limit of 25% based on the S*-to-T* ratio of 1:3.
The use of a phosphorescent compound can increase the internal quantum efficiency to 100% in theory. In other words, emission efficiency can be four times as much as that of the fluorescence compound. Therefore, the light-emitting element using a phosphorescent compound has been actively developed in recent years in order to achieve a highly efficient light-emitting element.
As a phosphorescent compound, an organometallic complex including iridium or the like as a central metal has particularly attracted attention because of its high phosphorescence quantum efficiency. An example of an organometallic complex exhibiting red light emission is an organometallic complex in which a pyrazine derivative is ortho-metalated with an ion of a Group 9 or Group 10 metal (see Patent Document 1, for example).